Bismaleimides are well known to be important thermosetting resin systems that have developed at a rapid rate during the last decade. Traditionally, epoxies have been the major resin chosen for advanced composites and adhesives. However, the limitation with epoxies is that their upper temperature range for structural performance is restricted to approximately 180.degree. C. in dry and 110.degree. C. in wet atmospheres. Higher temperature performance resins are needed for composites in applications where epoxies cannot be used, but unfortunately as high temperature properties are increased, the ease of processability of the resins is often reduced. Bismaleimides are preferred as matrix resins for composites over epoxies when high temperature resistance, good hot-wet environmental stability and improved fire, smoke and toxicity properties are required.
One of the advantages of most bismaleimide resins is their high glass transition temperature, which is required for many aerospace applications. The methylene dianiline based systems, although considered widely to be the work horse of the BMI industry have been found to be both extremely brittle and to undesirably contain quantities of the precursor carcinogenic methylene dianiline in the polymerization mixtures. This large degree of brittleness no doubt results from the high crosslink density due to the short bismaleimide segments, as well as crosslinks via the benzylene methylene unit. The brittle bismaleimide networks result in composites with microcracks and low damage tolerance. Bismaleimides derived from aromatic diamines are crystalline compounds with relatively high melting points. Unfortunately, the high melting temperature of the uncured bismaleimide results in a narrow processing window. Once the bismaleimide resin melts, it immediately begins to cure, making processing difficult for the neat resins. One means to overcome this drawback is to copolymerize the bismaleimide with molar deficiencies of comonomers such as diamines via the Michael addition reaction. Such chain extension improves the processability and reduces the melting point and the inherent brittleness of the bismaleimides. In practice, bismaleimides are prereacted with an aliphatic branched diamine to produce a lower melting of even liquid precursor, which has a wider thermal processing window relative to its flow behavior and network formation. It would be desirable to produce a somewhat tougher BMI system which would again cure without any volatiles eliminated that would be based upon less environmental hazardous, preferably single component precursors. It is also desirable to prepare systems that can liquefy at relatively low temperatures and possess an adequate processing window. Materials showing improved flame retardancy are also sought. These needs have led us to investigate new BMI chemistry.
A wide variety of bismaleimides have been prepared with the aim of tailoring specific resin properties by simply changing the structure and molecular weight of the diamine used for the synthesis. Some literature and patent disclosures considered relevant to the subject matter claimed herein are the following:
1. U.S. Pat. No. 4,276,344 to R. A. Forsch et al. illustrate bisimide formation using a phosphorus-containing aromatic diamine in which the bridging phosphorus moiety has the formula ##STR1## where R is methyl, ethyl or phenyl; and
2. I. K. Varma et al., in J. Macromol. Sci-Chem., A19(2), pp. 209-224 (1983) show trisimide and bisimide monomers derived from tris(m-aminophenyl) phosphine oxide by reaction with the selected anhydride.